JP3525905B2 - Method for producing structural steel with excellent toughness in weld heat affected zone - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a structural steel material which is suitable for use in ships, buildings, marine structures, steel pipes, storage tanks and the like, and which has excellent toughness in a weld heat affected zone.

[0002]

2. Description of the Related Art In a heat-affected zone of a steel sheet, austenite grains are coarsened during heating and cooling, and the structure after cooling is also coarsened to deteriorate toughness. Therefore, for example, Japanese Patent Publication 55-
As disclosed in Japanese Patent No. 26164, fine Ti is contained in steel.
A method of suppressing the growth of austenite grains and improving toughness by dispersing N is widely adopted.

As described above, when utilizing TiN, it is necessary to secure a sufficient amount of TiN for refining the austenite grains. At the same time, however, surface crack prevention of the continuously cast slab and heat-affected zone due to solid solution N are involved. From the viewpoint of preventing the deterioration of toughness of, for example, in E class steel for shipbuilding for large heat input welding,
Ti content is 0.01 to 0.02 mass%, N content is 0.003 to 0.005 mass%
The degree of composition is designed.

[0004]

However, even when the addition amounts of Ti and N are controlled within a predetermined range,
When the cooling conditions at the time of casting are changed, there is a problem that the toughness values in the weld heat affected zone of the manufactured steel sheet vary. An object of the present invention is to advantageously solve the above problems and provide a structural steel material having excellent toughness in the weld heat affected zone.

[0005]

Means for Solving the Problems In order to solve the above-mentioned problems, the inventors have extensively studied the grain size distribution of TiN, the steel composition, and the cooling conditions during casting. I obtained the findings. a) In order to prevent coarsening of austenite grains at the time of heating, ensure that Ti and N contents are not dissolved even at a high temperature, and set the initial average grain size (circle equivalent diameter) of TiN to 0.02 to 0.04.
It is effective to set in the range of μm. b) The distribution state of TiN in the base material can be controlled by appropriately controlling the cooling rate of the slab according to the Ti / N ratio. The present invention is based on the above findings.

That is, the gist of the present invention is as follows. 1. % By mass, C: 0.01 to 0.18%, Si: 0.02 to 0.60%,
Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.015% or less, Al: 0.005 to 0.100%, Ti: 0.007 to 0.030% and N: 0.0040 to 0.0100%, and the balance Fe and unavoidable impurities. Cooling time t from 1500 ° C to 1100 ° C averaged over the entire thickness of the slab when cooling the slab with the composition
_{Regarding 15/11} (seconds) and Ti / N ratio, the following equation (1) 22600 / (t _15/11 ) ^1.25 ≤ Ti / N ≤ 1818000 / (t _15/11 ) ^1.7 --- (1) A method for manufacturing a structural steel material having excellent toughness in a heat-affected zone of a weld, which is characterized by satisfying.

2. In the above 1, the steel material further contains Cu: 0.02 to 1.5%, Ni: 0.02 to 0.6%, and Cr: 0.05 to
0.50%, Mo: 0.02 to 0.50%, Nb: 0.003 to 0.030% and V: 0.03 to 0.15% of the welding heat-affected zone characterized by having a composition containing one or more selected from A method for manufacturing a structural steel material having excellent toughness.

3. In the above 1 or 2, the steel material further contains B: 0.0002 to 0.0020% and REM: 0.001 to 0.0% by mass.
1 selected from 20% and Ca: 0.001 to 0.010%
A method for producing a structural steel material having excellent toughness in a heat-affected zone of a weld, which is characterized by having a composition containing one or more kinds.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason why the component composition of the raw material is limited to the above range in the present invention will be described. In addition, all of the component percentages shown below are "mass%". C: 0.01 to 0.18% C is an element useful for improving the strength, and 0.01% or more is necessary to secure the strength of the base material, but 0.18%
If contained in excess of 10%, the toughness and weldability will deteriorate, so
The C content was limited to the range of 0.01 to 0.18%. The practically suitable range is 0.06 to 0.14%.

Si: 0.02 to 0.60% Si is an element useful for increasing strength, but its content is 0.02%.
If less than 0.60%, the effect of addition is poor, while if over 0.60%, the toughness of the weld heat-affected zone deteriorates significantly, so the Si content is 0.02%.
It was limited to the range of ~ 0.60%.

Mn: 0.60-2.00% Like Si, Mn is also an element useful for increasing strength, but if the content is less than 0.60%, its effect of addition is poor, while 2.00%.
If it exceeds 1.0, the toughness of the base material deteriorates significantly, so the Mn content is 0.
Limited to the range of 60-2.00%. In addition, preferably from 1.00
The range is 1.70%.

P: 0.030% or less P is an element that deteriorates toughness, so it is preferable to reduce it as much as possible, but 0.030% is acceptable.

S: 0.015% or less S is mainly present as MnS in steel and has an action of refining the structure of the rolled structure and the weld heat affected zone. However, if the content exceeds 0.015%, the toughness of the base material deteriorates, so 0.015% was made the upper limit.

Al: 0.005-0.100% Al requires the addition of at least 0.005% for deoxidation, but if it exceeds 0.100%, the deoxidizing effect saturates and rather raises the cost. It was made to contain in 0.005 to 0.100% of range.

Ti: 0.007 to 0.030% Ti is mainly present as TiN and is an element essential for refining crystal grains. TiN is stable even at high temperatures and has an effect of suppressing the growth of austenite grains in the heat affected zone of welding. At the time of welding, in order to obtain this effect even in the vicinity of the melting line, it is necessary to secure a sufficient amount of TiN even in a high temperature region of 1400 ° C. or higher, and for this purpose, Ti of at least 0.007% is required. On the other hand, if the Ti content exceeds 0.030%, the cleanliness and toughness of the steel deteriorate. Therefore, the Ti amount is 0.007 to 0.030.
It was limited to the range of%.

N: 0.0040 to 0.0100% N combines with Ti to form TiN, which effectively contributes to the refinement of crystal grains. As described above, TiN has the effect of suppressing the growth of austenite grains in the heat-affected zone of the weld. To obtain this effect near the weld line during welding, 1400 ° C
It is necessary to secure a sufficient amount of TiN even in the above high temperature range,
Therefore, N of 0.0040% or more is required. on the other hand,
When the amount of Ti exceeds 0.0100%, it exists in a solid solution state during welding heating and reduces the toughness of the heat affected zone. Therefore, the amount of N is 0.00
It was limited to the range of 40-0.0100%.

Although the essential components have been described above, the following components can be appropriately contained in addition to these essential components in the present invention. Cu: 0.02 to 1.5%, Ni: 0.02 to 0.6%, Cr: 0.05 to 0.50
%, Mo: 0.02 to 0.50%, Nb: 0.003 to 0.030%, and V: 0.03 to 0.15%. One or more selected from Cu, Ni, Cr, Mo, Nb and V are all hardenable. Is an element that improves the strength of the base material by improving Cu. However, in order to obtain this effect, Cu ≧ 0.02%, Ni
≧ 0.02%, Cr ≧ 0.05%, Mo ≧ 0.02%, Nb ≧ 0.003%, V
Addition of ≧ 0.03% is necessary. On the other hand, when Cu exceeds 1.5%, the hot workability deteriorates significantly, so 1.5% was made the upper limit. If Ni exceeds 0.60%, the manufacturing cost rises, so 0.60% was made the upper limit. When Nb exceeds 0.030%, the toughness of the heat-affected zone deteriorates, so 0.030% was made the upper limit. Regarding Cr, Mo and V, addition of a large amount causes weldability,
It deteriorates toughness, so 0.50%, 0.50%, 0.15%
% Was set as the upper limit.

B: 0.0002 to 0.0020%, REM: 0.001 to 0.
020% and Ca: 1 selected from 0.001 to 0.010%
All or one or more of B, REM and Ca are elements that effectively contribute to the refinement of ferrite grains after rolling. That is, B is
Suppresses the formation of coarse ferrite by segregating at the grain boundaries,
It has the effect of refining the ferrite grain size after rolling. This effect is recognized by addition of 0.0002% or more, but addition of more than 0.0020% reduces the toughness of the base material, so the range was made 0.0002 to 0.0020%. REM and Ca each form a stable fine oxide at high temperature, thereby making the ferrite grains after rolling fine. Further, it also has the effect of improving the toughness of the weld heat affected zone. These effects are observed when REM and Ca are added at 0.001% or more.
On the other hand, if REM and Ca are added in large amounts, the cleanliness of the steel and the toughness of the base material decrease, so 0.020% and 0.0
The upper limit was 10%.

In the present invention, the initial average grain size (circle equivalent diameter) of TiN contained in the manufactured steel material is 0.02 to 0.
It is important to set it to 04 μm. This is because the finer the TiN, the higher the density, so it is effective for refining the austenite grains during heating. However, the smaller the diameter, the faster the dissolution rate at high temperature, and most of the average grain size is less than 0.02 μm. TiN melts during welding, so a sufficient effect cannot be obtained,
On the other hand, when the average particle size is larger than 0.04 μm, the TiN density becomes low and the sufficient austenite grain refining effect cannot be obtained.

In order to set the average particle size of TiN to 0.02 to 0.04 μm, it is necessary to control the cooling rate during solidification in addition to the addition amounts of Ti and N. FIG. 1 shows the Ti / N ratio and the cooling rate on the TiN average grain size of a steel sheet hot-rolled after solidifying 5 kinds of steel satisfying the compositional range of the present invention with 4 kinds of cooling patterns. This is the result of an investigation on the impact. The average particle size of TiN was determined by electrolytically etching the surface of the steel sheet, then observing with SEM, and performing image analysis of the SEM image. In addition, as an index of cooling rate, Ti
A cooling time t _15/11 from 1500 ° C. to 1100 ° C. at which N precipitation and growth occurs is used.

As shown in the figure, the average particle size of TiN is
It can be seen that it greatly changes depending on the Ti / N ratio and the cooling rate. If t _15/11 is constant and the Ti and N contents satisfy the proper range of the present invention, the average grain size of TiN is Ti / N.
It was also clarified that the ratio was almost uniquely determined, and therefore the Ti / N ratio was the optimum index for grain size control.

Next, referring to FIG. 2, the average particle size: 0.02 μm and
The results of plotting the cooling rate and the Ti / N ratio giving 0.04 μm are shown. From the results of FIG, Ti / N ratio gives a TiN average particle size of 0.02μm: (Ti / N) ₂₀ by the following equation (2) (Ti / N) 20 = 22600 / (t 15/11) 1.25 - -At (2), Ti / N ratio giving TiN average particle size of 0.04 μm:
It was found that (Ti / N) ₄₀ is expressed by the following equation (3) (Ti / N) ₄₀ = 1818000 / (t _15/11 ) ^1.7 --- (3). Therefore, in order to control the average particle size within the range of 0.02 to 0.04 μm, Ti / N
It is understood that the ratio and the cooling time t _15/11 from 1500 ° C. to 1100 ° C. should be controlled within a range that satisfies the relationship of the following expression (1). 22600 / (t _15/11 ) ^1.25 ≤ Ti / N ≤ 1818000 / (t _15/11 ) ^1.7 --- (1)

In the past, the higher the cooling rate, the more Ti
It has been known that the average grain size of N becomes finer. However, in the conventional steel sheet, even if the composition and the cooling rate are individually controlled within predetermined ranges, the optimum TiN grain size cannot be obtained. It is considered that the reason was that the composition and the cooling rate were not controlled at the same time.

As described above, the Ti / N ratio and t
If you also control _15/11 , the TiN average particle size will be _0.02-0.04.
It can be controlled in the range of μm, and as a result, it effectively suppresses coarsening of austenite grains during heating,
It is possible to secure the toughness of the heat affected zone. The reason for this is not clear yet, but it is considered as follows. That is, TiN during cooling
Since the growth rate of Ti is controlled by the diffusion of Ti, the larger the amount of Ti, the larger the size. However, in the composition range of the present invention, if the cooling rate and the Ti / N ratio are controlled to be constant, almost the same average grain size is obtained. The diameter can be given. This is because the growth of TiN increases as the Ti content increases, while the TiN density increases as the N content increases (the Ti / N ratio decreases) and the size per unit decreases when the Ti content is constant. It is estimated that there is. Therefore, by controlling not only the addition amounts of Ti and N but also the Ti / N ratio in addition to the control of the cooling rate, it is possible to obtain a predetermined TiN grain size in a wider component range and cooling conditions. Conceivable.

Next, a method for manufacturing the steel material of the present invention will be described. Steel satisfying the above preferable composition range is melted by a converter, electricity, etc. and solidified by a continuous casting method or an ingot method. At this time, the cooling time t _15/11 from 1500 ° C. to 1100 ° C. is adjusted according to the Ti / N ratio, and cooling is performed under the conditions that satisfy the above expression (1). If it is difficult to change the cooling time during operation, the Ti / N ratio may be adjusted to an appropriate range according to the cooling conditions. Furthermore, the slab is rolled to a predetermined thickness, which is usually done during rolling12
When heated below 00 ° C for several hours, the distribution state of TiN hardly changes. As described above, the Ti / N ratio and 15
By properly controlling the cooling time t _15/11 from 00 ° C to 1100 ° C, the TiN average grain size can be set to 0.02 to _0.04 μm, and as a result, a steel material having excellent toughness in the heat affected zone can be manufactured. be able to.

[0026]

EXAMPLE Steels having the composition shown in Table 1 were melted in a converter and made into slabs having a thickness of 200 mm and 300 mm by a continuous casting method. For each slab, 1500 ° C to 11 at solidification
As a result of calculating the cooling time t _15/11 up to 00 ° C. on the _basis of the casting speed, the water cooling condition, etc., and averaging in the thickness direction, the thickness:
200 mm slab is 1900 s, thickness: 300 mm slab is 3400 s
It was s. Under each cooling condition, the Ti / N ratio ((Ti /
N) ₂₀ and (Ti / N) ₄₀ ) were calculated from the equations (2) and (3). Further, these slabs were finished by heating and rolling to a plate thickness of 50 mm, TiN distribution measurement samples were taken from 1/4 of the plate thickness, and the average particle size of TiN was determined by SEM. Furthermore, from the plate thickness 1/4 part in the direction perpendicular to the rolling direction, 12 mm × 75 mm ×
80mm test piece is sampled and heat input by high frequency heating device: 40
After applying a heat cycle (maximum heating temperature 1400 ° C) corresponding to the heat affected zone near the fusion line of 0 kJ / cm electrogas arc welding, a Charpy impact test piece was sampled at -40 ° C
Charpy absorbed energy (vE-40) was measured. The obtained results are shown in Table 2.

[0027]

[Table 1]

[0028]

[Table 2]

As is clear from Table 2, the steel material obtained according to the present invention has a TiN average grain size of the base material of 0.02 to 0.04 μm.
Is satisfied, and high toughness is obtained in the weld heat affected zone. On the other hand, those that do not satisfy the requirements of the present invention have low toughness in the heat-affected zone. In particular, even if the component composition satisfies the proper range of the present invention, if the Ti / N ratio and cooling conditions do not satisfy the conditions of the present invention (No. 11 to 16),
Good toughness was not obtained because the TiN was refined or coarsened too much.

[0030]

As described above, according to the present invention, it becomes possible to produce a steel material having a good toughness even in the heat-affected zone having a large heat input, with a good yield, and to remarkably improve the safety of various welded structures. be able to.

[Brief description of drawings]

Fig. 1 TiN cooling condition and Ti influence on TiN average grain size
It is a figure which shows the influence of / N ratio.

FIG. 2 shows the cooling conditions of the slab that affect the TiN average grain size and
It is a figure which shows the influence of Ti / N ratio.

─────────────────────────────────────────────────── ─── Continuation of front page (56) Reference JP-A-3-264614 (JP, A) JP-A 64-5644 (JP, A) JP-A 9-104946 (JP, A) JP-A 9- 24448 (JP, A) JP-A-61-113715 (JP, A) JP-B-55-26164 (JP, B1) (58) Fields investigated (Int.Cl. ⁷ , DB name) B22D 11/22 B22D 11 / 00 C22C 38/00 301 C22C 38/14 C22C 38/58

Claims (3)

(57) [Claims] 1. In mass%, C: 0.01 to 0.18%, Si: 0.02 to 0.60%, Mn: 0.60 to 2.00%, P: 0.030% or less, S: 0.015% or less, Al: 0.005 to 0.100%, Ti : 0.007 to 0.030% and N: 0.0040 to 0.0100% with the balance being Fe and unavoidable impurities in the composition. When cooling the slab, 1500 ° C averaged over the entire thickness of the slab.
From the cooling time t _15/11 (in seconds) and Ti / N ratio of up to 1100 ° C., the following equation _{(1) 22600 / (t 15/11} ) 1.25 ≦ Ti / N ≦ 1818000 / (t 15/11) 1.7 - --A method for producing a structural steel material having excellent toughness in a heat-affected zone of a weld, which satisfies the relationship of (1). 2. The steel material according to claim 1, further comprising, by mass%, Cu: 0.02 to 1.5%, Ni: 0.02 to 0.6%, Cr: 0.05 to 0.50%, Mo: 0.02 to 0.50%, Nb: 0.003 to 0.030. % And V: 0.03 to 0.15%, a composition containing one or more selected from the group consisting of 0.03 to 0.15%, and a method for producing a structural steel material excellent in toughness of a weld heat affected zone. 3. The steel material according to claim 1 or 2, further comprising one or more selected from B: 0.0002 to 0.0020%, REM: 0.001 to 0.020% and Ca: 0.001 to 0.010% in mass%. A method for producing a structural steel material excellent in toughness of a heat-affected zone of a weld, characterized by having a composition containing.